1. Field of the Invention
The present invention relates to a commercially advantageous process for preparing 3,4,5,6-tetrahydrophthalimide (hereinafter referred to as ".DELTA..sup.1 -THPI").
2. Prior Art
.DELTA..sup.1 -THPI is an industrial material useful as the intermediate for preparation of insecticides, herbicides or like agricultural chemicals.
Conventional processes for preparing .DELTA..sup.1 -THPI include the following processes.
(1) A process comprising heating 3,4,5,6-tetrahydrophthalic anhydride (hereinafter referred to as ".DELTA..sup.1 -THPA") and urea to a temperature of 155.degree. to 160.degree. C. and recrystallizing the obtained reaction product from benzene for separation (J. Chem. Soc., 4846, (1952)): PA0 (2) A process comprising reacting .DELTA..sup.1 -THPA with aqueous ammonia at a temperature of 110.degree. to 160.degree. C. (Khim. Farm. Zh., [3], 28 (1969)): PA0 (3) A process comprising reacting .DELTA..sup.1 -THPA with an ammonia gas at a temperature of 145.degree. to 152.degree. C. (U.S.S.R. Inventor's Certificate No.859,363). PA0 (i) reacting a crude 3,4,5,6-tetrahydrophthalic anhydride containing as impurities at least one carboxylic anhydride (A) selected from the group consisting of phthalic anhydride, hexahydrophthalic anhydride and structural isomers of 3,4,5,6-tetrahydrophthalic anhydride with an alcohol represented by the formula EQU R(OH).sub.n (I) PA0 (ii) adding ammonia to the reaction mixture obtained in the step (i) to subject the 3,4,5,6-tetrahydrophthalic anhydride and if any, the monoester thereof to a reaction for conversion into 3,4,5,6-tetrahydrophthalimide.
However, the foregoing processes require the use of a high-purity .DELTA..sup.1 -THPA as the starting material in order to obtain a high-purity .DELTA..sup.1 -THPI in high yields.
For the production of .DELTA..sup.1 -THPA serving as the starting material, we have proposed methods in which 1,2,3,6-tetrahydrophthalic anhydride (hereinafter referred to as ".DELTA..sup.4 -THPA") is isomerized (Japanese Examined Patent Publications No. 54144/1983 and No.54148/1983).
These methods for preparing .DELTA..sup.1 -THPA, although commercially advantageous, give a reaction product containing about 1 to about 10% of carboxylic anhydrides (hereinafter referred to as "contaminating phthalic anhydrides") such as hexahydrophthalic anhydride, phthalic anhydride and .DELTA..sup.4 -THPA (i.e. 1,2,3,6-tetrahydrophthalic anhydride) or like structural isomer of .DELTA..sup.1 -THPA.
When ammonia is acted on such starting material comprising .DELTA..sup.1 -THPA as the main component and the contaminating phthalic anhydrides in the conventional manner, not only the .DELTA..sup.1 -THPA but also the contaminating phthalic anhydrides are converted into the corresponding imides. These imides can not be easily separated from one another and thus necessitate a separation procedure for isolation. Consequently a high-purity .DELTA..sup.1 -THPI can not be produced in high yields.